Field of the Invention
The present invention relates to a lane keeping assist apparatus for assisting a driver in driving a vehicle to keep in lane.
Description of the Related Art
Conventionally, a lane keeping assist apparatus which performs lane keeping assist control has been known (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2009-18626). Such a lane keeping assist apparatus detects white lines on the left and right sides of a roadway using, for example, a camera sensor, and sets a target travel line within a lane (e.g., a line located at the midpoint between the left-hand and right-hand white lines) on the basis of the left-hand and right-hand white lines. The lane keeping assist apparatus calculates a target steering angle on the basis of the amount of deviation of the travel position (in the width direction) of a vehicle on which the apparatus is mounted from the target travel line and the angle of deviation of the heading direction of the vehicle with respect to the target travel line, and sends to a steering control apparatus a command signal which represents a target steering torque set on the basis of the target steering angle.
The steering control apparatus drives a motor in accordance with the target steering torque represented by the command signal from the lane keeping assist apparatus, to thereby cause a steering mechanism to generate the target steering torque. Thus, the steering angle is controlled such that the vehicle has a travel position near the target travel line. Accordingly, a driver can easily cause his/her vehicle to travel along a proper line within a lane.
In general, such a lane keeping assist apparatus performs the lane keeping assist control within a limited vehicle speed range; i.e., only during high speed travel. Therefore, a demand has arisen for expanding the vehicle speed range to the low speed side so as to perform the lane keeping assist control over the entire vehicle speed range. If the vehicle speed range within which the lane keeping assist control is performed is expanded to the low speed side in order to meet the demand, calculation of proper target steering torque becomes difficult during low speed travel for the following reason.
When the travel direction of the vehicle is to be changed, a lateral force (lateral acceleration) must be generated in the vehicle. When a steering force is applied to each steerable wheel, the tire of the steerable wheel has a slip angle. The lateral force is mainly generated by the tire. Specifically, the rubber of the tire deforms as a result of generation of the slip angle, and the rubber generates a restoring force to restore itself from the deformed state, whereby the lateral force is generated. However, during low speed travel, the slip angle becomes smaller than the slip angle generated during high speed travel. Also, self-aligning torque becomes hard to generate. Accordingly, the mechanism of generation of the lateral force changes between the case of high speed travel and the case of low speed travel, and thus, the relation between the applied steering torque and the generated lateral force changes between the two cases. Therefore, in the case where the target steering torque set on the basis of the target steering angle is used to control the motor during low speed travel in the same manner as that during high speed travel, steering torque which allows obtainment of proper lateral force cannot be generated. This results in a drop in lane keeping assist performance.
In the apparatus proposed in the Japanese Patent Application Laid-Open No. 2009-18626, a target lateral acceleration is calculated on the basis of the amount of deviation of the travel position (in the width direction) of a vehicle on which the apparatus is mounted from a target travel line, the angle of deviation of the heading direction of the vehicle with respect to the target travel line, and the curvature of the target travel line; and a target steering torque set on the basis of the target lateral acceleration is used as a feedforward control variable. In the apparatus, the actual lateral acceleration is calculated from the yaw rate of the vehicle detected by a yaw rate sensor, and a value obtained by multiplying the deviation of the actual lateral acceleration from the target lateral acceleration by a feedback gain is used as a feedback control variable. This apparatus calculates a final target steering torque by adding the feedforward control variable and the feedback control variable together.
However, the proposed apparatus also has the following problem. Since lateral force becomes hard to generate during low speed travel, even when the motor is controlled through use of the target steering torque as in the case of the high speed travel, proper lateral force cannot be generated, and lane keeping assist performance lowers.